This invention relates to water purification and more particularly it relates to an improved process for producing high purity water.
In using reverse osmosis, one problem is the scaling of the membrane resulting from deposition of materials such as calcium and magnesium compounds, for example, as well as other compounds and elements. Fouling of the membranes interferes with flux, greatly reducing the efficiency of the water purification system.
Another problem in producing high purity product water in a double pass reverse osmosis system is the difficulty in rejecting gases such as carbon dioxide and/or ammonia. Such gases pass through the membrane and re-establish an equilibrium in the permeate, adversely affecting product water resistivity.
The pH of feedwater to a double pass reverse osmosis is often controlled to a particular value to provide high resistivity water. However, a pH range of feedwater that produces high resistivity product water in one instance may not always produce high resistivity product water in another instance. That is, pH of feedwater to the first pass reverse osmosis has the problem that it does not always provide a control that produces high resistivity product water.
The presence of total alkalinity due mainly to bicarbonate, smaller amounts of carbonate, with small contributions by other ions and of carbon dioxide in the feedwater is responsible for significant changes in apparent rejection of salts and thus in the conductivity of product water from a double pass reverse osmosis system. As noted, reverse osmosis membranes are transparent to dissolved gases. Thus, CO.sub.2 present in the feed side of the first pass membrane passes through the membrane to the interpass while bicarbonate and carbonate comprising total alkalinity is mostly rejected along with other anions and cations. This results in a change in the total alkalinity:CO.sub.2 ratio, a loss of buffering capacity and causes a drop in pH from feed to interpass or permeate from the first membrane in a double pass reverse osmosis system. The same process is repeated from the interpass to the second pass product. The resulting change in the interpass pH can have the result of moving the interpass pH away from the pH which results in high resistivity product water from the second pass reverse osmosis unit. Thus, as noted, setting the pH of the feedwater to a fixed value for a double pass reverse osmosis does not always result in high resistivity product water.
In addition, when a particular pH is chosen for producing high resistivity water from a particular feedwater, changes in the feedwater composition, e.g., alkalinity, can render the chosen pH not optimum. Thus, lower quality product water results even though the feedwater has been maintained within a narrow pH range which was, at one time, thought to be optimum. Further, it will be appreciated that different membranes have the capacity to reject different ions to a lesser or greater extent. That is, some membranes reject anions better than cations and vice versa. When there is preferential rejection, there can be leakage of the other or opposite ion. pH of the feedwater has a large impact on the capacity of the particular membrane to reject the particular anion or cation. However, any membrane's performance can vary in a systematic way with pH to reach a peak value for rejection, and thereafter its performance declines on either side of an optimum pH.
This concept is illustrated in FIG. 2 where A and B denote the highest resistivity for a given pH value. On either side of these points, resistivity declines. FIG. 3 also illustrates that different pH values can result in the same quality product water. However, on either side of the pH value, product quality declines. "A" can represent TDS and "B" high alkalinity. Further, the process is complicated by membrane selection. The negatively charged membrane of Fluid Systems Inc., referred to by the tradename HRRX membrane, operates in a pH range of 6.5 to 8 with a 99.4% rejection, while Toray's positively charged membrane, having the designation SU910S, operates at a pH of 9 to 9.5 with a 99.5% rejection. The lower pH is better for removing ammonia and the higher pH is better for removing carbon dioxide. Thus, it will be seen that there is a great need for a process which can be operated in a way which avoids these problems.
Attempts at removing carbon dioxide to provide high resistivity water in the past have only been partially successful and often end up further contaminating the water. For example, U.S. Pat. No. 4,574,049 discloses a process for removing carbon dioxide and other impurities from a water supply using double pass reverse osmosis membranes. The process includes providing a first reverse osmosis until having an inlet, a product outlet and a brine outlet; providing a second reverse osmosis unit having an inlet, a product outlet and a brine outlet; locating the second reverse osmosis unit downstream of the first reverse osmosis unit with the product outlet of the first reverse osmosis unit being coupled to the inlet of second reverse osmosis unit; providing water to be purified to the inlet of first reverse osmosis unit; treating the product from the reverse osmosis unit at a location upstream of second reverse osmosis unit with a chemical treatment agent comprising a solution having a pH that exceeds 7 to reduce carbon dioxide concentration of the product by chemical conversion and to ionize certain otherwise difficult to remove chemicals; and directing the product from second reverse osmosis unit toward a point of use or storage for purified water.
However, this process which normally uses sodium hydroxide for increasing the pH results in the addition of sodium which, because of its small ionic radius, is difficult to remove by subsequent membranes. Further, the addition of sodium hydroxide has another disadvantage in that the series of reactions removing carbon dioxide are relatively slow when compared to reverse osmosis unit contact time. Thus, the effectiveness of the operation is limited by the sodium hydroxide reactions, and further, this process does not remove ammonia.
U.S. Pat. No. 5,338,456 discloses a water purification process for removing dissolved solids of the type that are normally present in a municipal or similar water supply. The process uses a forced draft decarbonator having an inlet and a product outlet, a vacuum degasifier having an inlet, a product outlet and a water level sensor, and a reverse osmosis unit having an inlet, a product outlet and a brine outlet. The vacuum degasifier is located downstream of the forced draft decarbonator with the product outlet of the forced draft decarbonator being coupled to the inlet of the vacuum degasifier. The reverse osmosis unit is located downstream of the vacuum degasifier with the product outlet of the vacuum degasifier being coupled to the inlet of the reverse osmosis unit. Water to be purified is provided to the inlet of the forced draft decarbonator at a predetermined rate. According to the invention, the rate at which water to be purified is a provided to the inlet of the forced draft decarbonator is a function of a predetermined water level in the vacuum degasifier.
Japanese Patent 4-22490 discloses a pre-stage reverse osmosis membrane module, a post-stage reverse osmosis membrane module and a hydrophobic porous membrane module, to which an aqueous alkali solution circulating line is attached in the permeate side. That is, Japanese Patent 4-22490 utilizes an alkali solution in the permeate side to remove dissolved carbon dioxide by chemical reaction. The hydrophobic porous membrane module is placed between the pre-stage module and the post-stage module and has pores capable of permeating only gases. An inert gas blowing pipe is installed to the alkali aqueous solution circulating line.
Japanese Patent 2-2802 discloses reverse osmosis separator membrane module and degassing membrane module arranged in treating water line in series. The degassing membrane is formed by a porous supporter layer and high molecular homogeneous layer or minute layer arranged on the supporter layer. Oxygen separating. coefficient of the degassing membrane is not less than 1.3.
U.S. Pat. No. 4,897,091 discloses that gases such as carbon dioxide may be separated from rich liquor (such as methanol containing carbon dioxide) by passage of gas through a membrane which is the reaction product of (i) a polyamine and (ii) a polyisocyanate or a poly (carbonyl chloride).
U.S. Pat. No. 5,078,755 discloses removing dissolved gas from liquid, which comprises bringing the liquid containing the gas dissolved therein into contact with a membrane, thereby causing the dissolved gas to selectively permeate the membrane. The membrane is a permselective, composite membrane composed of a porous support and a nonporous, active membrane of a synthetic resin formed on the porous support, or is a permeable membrane having such characteristics that the nitrogen gas permeation rate at 30.degree. C. is in the range from 7.times.10.sup.-4 to 2.times.10.sup.-2 Nm.sup.3 m.sup.2.multidot.h.multidot.atm and that the amount of permeated stream is 100 g/m.sup.2.multidot.h or less when 20.degree. C. water is supplied to the membrane under atmospheric pressure while maintaining the pressure on its permeate side at 40 mm Hg.
U.S. Pat. No. 5,106,754 discloses that total organic carbon (TOC) and total inorganic carbon (TIC) monitoring of water is useful in determining the water quality. Conventional TOC and TIC monitoring techniques are not zero gravity compatible. The addition of microporous hydrophobic bladders in combination with a non-dispersive infrared analyzer allow for a two-phase, liquid and gas, zero gravity compatible TOC monitoring technique.
U.S. Pat. No. 5,116,507 discloses a method of treating an aqueous liquor, such as effluent liquor formed during coal gasification. The method comprises subjecting the liquor to dephenolation and ammonia stripping treatment to remove phenolic compounds and "free" ammonia from the liquor and then subjecting the resulting liquor, which still contains ammonium compounds and thus "fixed" ammonia, to reverse osmosis treatment to produce a permeate which is substantially free from impurities, including fixed ammonia.
U.S. Pat. No. 5,250,183 discloses an apparatus for manufacturing ultra-pure water, characterized in that a decarbonator/degassor and a reverse osmosis equipment for pretreatment of supply water are installed in the upper stream of a multiple effect evaporator.
U.S. Pat. No. 5,254,143 discloses a diaphragm for gas-liquid contact comprising a membrane having two surfaces, at least one surface of the membrane is hydrophilic and surfaces of micropores present in the membrane are hydrophobic. The diaphragm is used in contact apparatus in which a liquid is contacted with the hydrophilic surface of the membrane and a gas is contacted with the other surface.
U.S. Pat. No. 5,306,427 discloses a process for the separation of one or more, more permeable components from one or more, less permeable components in a feed stream. The process suggests two membrane separation stages in series wherein the feed is introduced into the low pressure side of the first stage, the permeate stream from the first stage is compressed and introduced into the high pressure side of the second stage and wherein the non-permeate stream from the second stage is recycled to the high pressure side of the first stage.
U.S. Pat. No. 5,413,763 discloses a method and apparatus for the measurement of the total organic carbon (TOC) content of a liquid. The inorganic carbon in the liquid is converted into carbon dioxide and removed from it. At the same time, oxygen is added to the liquid. The liquid is then exposed to ultraviolet radiation and the organic carbon thereby oxidized.
Japanese Patent 4-176303 discloses a gas-permeable membrane module containing a hollow fiber-shaped hydrophobic gas-permeable membrane used to remove the gas dissolved in a liquid. The liquid is supplied from an inlet, passed through the inside of the membrane from the membrane opening and sent to the other end of the membrane. A carrier gas is introduced from an outlet, passed around the bundle of the membranes and discharged from an outlet. The outlet is connected under these conditions to a vacuum source such as a vacuum pump, hence the gas dissolved in the liquid permeates through the membrane to the outside, and degasification is performed with high efficiency.
In U.S. Pat. No. 5,156,739, it is disclosed that water to be purified and degassed is passed through a reverse osmosis step from which a pure water stream and a high pressure waste water stream are produced. The high pressure waste water is passed through an eductor to produce a vacuum. The pure water stream is passed into a first volume of a degassifier and the vacuum is directed to a second volume of the degassifier. The first and second volume of the degassifier are separated by a hydrophobic membrane.
U.S. Pat. No. 5,670,053 discloses a process for purifying water including removing cations, anions and carbon dioxide and/or ammonia from water feed stream to produce high purity water having a resistivity of greater than 1 megohm-cm comprising the steps of providing a water feed stream to be purified, the stream containing cations, anions and carbon dioxide and/or ammonia; introducing the water feed stream to a high pressure side of a first reverse osmosis membrane module; passing water through the first reverse osmosis membrane to provide a first retentate having cations and anions concentrated therein and a first permeate depleted in cations and anions and containing carbon dioxide and/or ammonia; adding the first permeate to a high pressure side, of a gas permeable hydrophobic membrane module; passing carbon dioxide and/or ammonia through the gas-permeable membrane from the first permeate in the high pressure side of the gas-permeable hydrophobic membrane to provide a carbon dioxide and/or ammonia permeate on a low pressure side of the hydrophobic membrane and to provide a carbon dioxide and/or ammonia depleted retentate thereby removing carbon dioxide and/or ammonia from the first permeate; transferring the carbon dioxide and/or ammonia depleted retentate to the high pressure side of a second reverse osmosis membrane; and further purifying the carbon dioxide and/or ammonia depleted retentate by passing at least a portion thereof through the second reverse osmosis membrane to provide a second retentate and a second permeate, the second permeate having low levels of carbon dioxide and/or ammonia and having a resistivity greater than 1 megohm-cm.
U.S. Pat. No. 5,766,479 discloses a process for purifying water by removing dissolved materials therefrom, the process capable of producing purified water having a resistivity in the range of 2 to 10 megohm-cm. The process comprises providing a water feed stream to be purified and adjusting the pH of the water feed stream to a basic alkaline water solution to drive the equilibrium of a first weakly ionized material to become ionized in the basic solution. The basic water solution is introduced to a high pressure side of a first reverse osmosis membrane module and water is passed through the first reverse osmosis membrane to provide a first retentate having ions therein from the first weakly ionized material concentrated therein and a first permeate depleted in ions from the first weakly ionized material. The pH of the first permeate is adjusted to an acidic water solution to drive the equilibrium of a second weakly ionized material to become ionized in an acidic solution. The acidic water solution is transferred to the high pressure side of a second reverse osmosis membrane and the acidic water solution is purified by passing at least a portion thereof through the second reverse osmosis membrane to provide a second retentate containing ions of the second weakly ionized material ionized in the acidic solution and provide a second permeate depleted in the ions from the second weakly ionized material, the second permeate having a resistivity in the range of 1 to greater than 10 megohm-cm.
In spite of these prior processes, there is still a need for an improved process for producing high purity water, particularly in a double pass membrane system.